Method and apparatus for eliminating audio feedback

ABSTRACT

The present invention is embodied in a method and apparatus for eliminating audio feedback which includes an active microphone coupled to an amplifier which transmits signals received at the active microphone to a parametric speaker for broadcasting. The apparatus comprises at least one transducer for detecting at least one sonic frequency and generating an electrical signal representative of the at least one sonic frequency, a processor for receiving the electrical signal and generating a first ultrasonic frequency which has modulated thereon the at least one sonic frequency, a parametric demodulator for recovering the at least one sonic frequency from the first ultrasonic frequency, and a speaker for directly emitting the at least one sonic frequency.

This patent application is a continuation-in-part of application Ser.No. 08/684,311, filed Jul. 17, 1996, now U.S. Pat. No. 5,889,870.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to audio amplification. Specifically, thepresent invention relates to a device and method for eliminatingundesirable audio feedback which occurs when audible sound travels froma loudspeaker to a live or active microphone, when the loudspeaker isamplifying audio signals received at the active microphone.

2. State of the Art

Audio feedback is more than a nuisance in state of the art audiosystems. This is because it can cost time and money to physicallyconfigure or reconfigure audio systems to reduce, but not eliminate, thecause of the problem. The audio systems which are vulnerable to audiofeedback are those which include an active microphone and anamplification system including loudspeakers, such as a public addresssystem. Audio feedback is a result of sound from the loudspeakersfeeding back into the active microphone. The phenomenon is manifested asa high pitched electronic squeal which typically rises in volume untiladjustments are made to the audio system to correct the problem.Corrective action typically includes turning down the volume so that thesound which is being propagated by the loudspeakers does not exceed anaudio feedback threshold at the microphone. Once the audio feedbackthreshold is exceeded, a feedback loop arises which requires thatcorrective action be taken to break the loop.

FIG. 1 shows a typical state of the art audio system 60 as describedabove. The audio system 60 is shown having two loudspeakers 62 toprovide capability for stereo broadcasting. However, only a singleloudspeaker 62 is necessary for the audio feedback to occur. The othercritical elements of the audio system 60 are the active microphone 64and an amplification/processing system 66. It should be readily apparentthat audio systems capable of live broadcasting of sound involvingdetection by a microphone can include many other system components. Forexample, the audio system 60 might be part of a karaoke system whichmixes recorded music with live input from a microphone before the mixedsound is emitted from loudspeakers. What is important to recognize isthat audio feedback is a result of an audio system having loudspeakersand an audio input, where the audio input picks up enough of the soundcoming from the loudspeakers to cause audio feedback. This phenomenon iswell understood by those skilled in the art of live audio systems.

To understand the present invention, it is also necessary to relate thepresent invention od the general acoustic speaker art and the ongoingeffort to reproduce sound in its purest form. In an earlier patentapplication under Ser. No. 08/684,311 of the same inventor, a detailedbackground of prior art in speaker technology using conventionalspeakers having radiating elements was reviewed and is herebyincorporated by reference.

There are several disadvantages which are inherent in such conventionalspeakers. The primary disadvantage is distortion arising from the massof the moving diaphragm or other radiating component. Related problemsarise from distortion developed by mismatch of the radiator elementacross the spectrum of low, medium and high range frequencies--a problempartially solved by the use of combinations of woofers, midrange andtweeter speakers.

Attempts to reproduce sound without use of a moving diaphragm of largedisplacement include technologies embodied in parametric speakers,acoustic heterodyning, beat frequency interference and other forms ofmodulation of multiple frequencies to generate a new frequency. Intheory, sound is developed by the interaction in air (as a nonlinearmedium) of two ultrasonic frequencies whose difference in value fallswithin the audio range. Ideally, resulting compression waves would beprojected within the air as a nonlinear medium, and would be heard aspure sound. Despite the ideal theory, general production of sound forpractical applications has alluded the industry for over 100 years.Specifically, a basic parametric or heterodyne speaker has not beendeveloped which can be applied in general applications in a manner suchas conventional speaker systems. However, there are several patents andmatters known to those skilled in the art which demonstrate attempts atcreating a parametric speaker system.

For example, a publication by Robert T Beyer of Brown University in 1856noted the research of H. von Helmholtz commenting on "combinationtones--that do not come directly from the sound sources but that arisesecondarily through the interaction of the two primary tones." Ann.Phys. Chem. 99:497+ (1856). These early observations noted that both sumand difference tones were developed, giving rise to theoreticalquestions of cause. This phenomenon, known in music as Tartini tones,was originally postulated to be a form of beat frequency arising as adifference frequency between two original audible frequencies. In themid 1800's Helmholtz discovered the presence of the "sum" frequency,suggesting that the phenomenon might be nonlinear. Nevertheless, littleprogress developed with respect to sum and difference tones as anacoustic phenomenon until the early 1900's.

In 1921, U.S. Pat. No. 1,616,639 disclosed an application of two soundwaves of different frequencies simultaneously impressed on a vibratingbody (the ear) to form new waves equal to the sum and difference of thetwo interacting frequencies. The perceived application of this systemwas as part of an auditorium speaker system as shown in prior art inFIG. 2. Specifically, the reference suggests that a carrier frequency"C" can be amplitude modulated with a sound signal "S" to generate twosideband frequencies C+S and C-S. One of the sideband frequencies (i.e.C-S) is then filtered out, with the remaining sideband (C+S) beingtransmitted toward an audience 2 from the front end of the auditorium.The carrier frequency C was transmitted from a separate speaker 24 atthe opposite end of the auditorium. As these two opposing wave frontsarrive at an audience member, the modulated "difference" wave S issupposedly developed by the ear based on the concurrent imposition ofthe two high frequencies on the tympanic membrane.

In 1931, U.S. Pat. No. 1,951,669 was issued, teaching a similar conceptbased on the theory of heterodyning or interference of super-audible airwaves. This early system is represented in prior art FIG. 3 andcomprises two opposing loud speakers 46 and 47 at opposite ends of aroom, each being operated at separate super-audible frequencies 44 and45. By directing these speakers toward an intermediate region of air, anaudible signal was to be formed, based on the difference between theinterfering frequencies. However, those skilled in the art are unable todevelop meaningful sound from such a configuration where opposingspeakers are directed toward each other as illustrated in FIG. 2.

An additional line of thought developing this general theory of soundproduction is reflected in an article by Peter J. Westervelt,"Parametric Acoustic Array", published in The Journal of the AcousticalSociety of America, Vol 35, No 4, April 1963. This disclosure, which isadmittedly theoretical as opposed to experimental, attempts tomathematically define the requirements for generation of a differencewave as part of a parametric speaker system. After discounting earlierefforts to generate sound when opposing waves intersect at nonzeroangles, the article relies on the assumption that if the two separatesound sources can generate two beams of sound which are (i) perfectlycollimated, (ii) superimposed and (iii) with a beam of sound so narrowas to constitute a "line" along the axis of the primary beams, soundgeneration could be achieved. (Page 535, col 2) The system requires useof "a microphone in the carrier beam, and the output of the microphonemust then be fed into a conventional radio set in order to demodulatethe signal." Id. P 537. Here again, no practical application of thistheoretical study has developed, suggesting the absence of one or morekey teachings. Neither is any reference made to feedback control orspecial characteristics of a parametric speaker system in this domain.

After this non-exhaustive but revealing history into the development ofthe parametric speaker, it is important to note that a commercialquality parametric speaker system which utilizes the phenomenon ofacoustical heterodyning was created by the present inventor. In additionto the patent application of the present invention already mentionedabove, other patent applications have also been filed by the inventorwhich teach further refinements of the technology. During the course ofinvestigative research in this general field of parametric or acousticheterodyning technology, the present inventor discovered anunanticipated property of parametric output which relates to theproblems of audio feedback.

With this background as described above, it is observed that there is aserious present need for an audio system which is not vulnerable to thedisruptive and costly effects of such audio feedback. It would be asignificant advancement to have an audio system which provides soundreproduction without the frustrating potential of audio feedback withinthe system.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod and apparatus for eliminating audio feedback from an audio systemincluding an active audio input, an amplifier, and a speaker.

It is another object to provide a method and apparatus for eliminatingaudio feedback from an audio system which includes a parametric speaker.

It is another object to provide a method and apparatus for eliminatingaudio feedback from an audio system which includes heterodyne processingof audio input, and a conventional speaker for broadcasting theprocessed audio input.

The preferred embodiment of the present invention includes an activemicrophone coupled to an amplifier which transmits signals received atthe active microphone to a parametric speaker for broadcasting. In thepreferred embodiment, the principle of operation is based on eliminatingaudio feedback by utilizing a parametric speaker to broadcast thesignals.

In another aspect of the invention, it follows that a conventionalspeaker system can still be used and audio feedback still eliminated ifprocessing of the signals from the active microphone occurs utilizingthe same signal processing which occurs in the audio system whichincludes a parametric speaker.

These and other objects, features, advantages and alternative aspects ofthe present invention will become apparent to those skilled in the artfrom a consideration of the following detailed description taken incombination with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the minimal system components of a priorart audio system which is vulnerable to the effects of audio feedback.

FIG. 2 is a block diagram of a prior art figure consolidated from FIGS.1 and 2 in U.S. Pat. No. 1,616,639.

FIG. 3 is a block diagram of a prior art figure extracted from U.S. Pat.No. 1,951,669 and corresponds to FIG. 5 of the referenced patent.

FIG. 4 is a block diagram of the preferred embodiment of the presentinvention showing the minimal audio system components which function tothereby eliminate audio feedback.

FIG. 5 is a block diagram which provides a detailed description of thecomponents of the processing circuitry in the preferred embodiment.

FIG. 6 is a block diagram illustrating an alternative embodiment whichincludes an active microphone, the processing circuitry of theparametric speaker system shown in FIG. 5, and a conventional speakerinstead of the parametric speaker of the preferred embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made to the drawings in which the various elementsof one preferred embodiment of the present invention will be givennumerical designations and in which the preferred embodiment of theinvention will be discussed so as to enable one skilled in the art tomake and use the invention.

FIG. 4 shows in a block diagram that the preferred embodiment of thepresent invention is embodied in an audio system 70 which includes anactive (live) microphone 72, amplification and processing circuitry 74and at least one parametric speaker 76. It is well understood by thoseskilled in the art that the minimal system 70 can be expanded to includeother system components. These other system components include, amongother things, mixers, receivers, compact disc players, and tape playersof various kinds. In other words, input to the audio system 70 can comefrom many different sources and recordable media. However, theadvantages of the present invention are recognized only when at leastone source of audio input is an active microphone 72.

The minimal system 70 is illustrated to show the critical elements whichwhen combined, result in an audio system 70 which is not subject to theeffects of audio feedback. To demonstrate elimination of audio feedback,the physical configuration of the audio system components 72, 74 and 76should be arranged such that the microphone 72 is disposed in front ofthe parametric speaker 76. It should also be obvious that in normal use,however, the audio system 70 should be disposed in a physicalconfiguration which serves the purposes of a live broadcast where themicrophone 72 will typically be disposed at a podium or on a stage, andthe parametric speaker 76 positioned so that an audience will hear asmuch sound as possible. Therefore, a face 78 of the parametric speakeris typically turned toward an audience or beamed toward a reflectiveelement which redirects emitted sound to the audience.

An explanation of the operation of the audio system 70 and how audiofeedback is eliminated is as follows. The microphone 72 is activelyfunctioning as a transducer, receiving an audio signal 82 and generatingelectrical signals 80 which are representative of the received audio 82.The detailed function of the microphone 72 is well known to thoseskilled in the art and further explanation is unnecessary.

The electrical signals 80 are transmitted to processing circuitry 74.After processing, the electrical signals 80 are transmitted to theparametric speaker 76 for emission therefrom as an audible signal 84.The audible signal 84 from the parametric speaker 76 is typically theoriginal audio signal 82 which has been amplified. It should be obvious,however, that the original audio signal 82 can be manipulated in otherways which do not alter the basic operations of the present invention.

The processing of the electrical signals 80 for emission via theparametric speaker 76 depends upon several factors.

For example, in the preferred embodiment, a single parametric speaker 76is used to emit the audible signal 84. In order for the singleparametric speaker 76 to operate as described requires an understandingof acoustical heterodyning and parametric speakers. A detailedunderstanding can be obtained through the materials disclosed in U.S.patent application Ser. Nos. 08/684,311 and 08/744,114 which are herebyincorporated by reference.

A brief explanation of the signal processing which occurs in theprocessing circuitry 74 is nevertheless provided. FIG. 5 provides adetailed description of the components of the processing circuitry 74 inthe preferred embodiment. This apparatus 74 comprises an oscillator ordigital ultrasonic wave source 90 for providing a base or carrier wave91. This wave 91 is generally referred to as a first ultrasonic wave orprimary wave. An amplitude modulating component 92 is coupled to theoutput of the ultrasonic generator 90 and receives the base frequency 91for mixing with the audible input signal 82. The sonic input signal 82may be supplied in either analog or digital form, and could be music orvoice from any conventional signal source such as the active microphone72 or other form of sound. If the input signal 82 includes upper andlower sidebands as shown in FIG. 5, a filter component may be includedin the modulator to yield a single sideband output on the modulatedcarrier frequency, depending on the frequency output desired.

A transducer or parametric speaker 76 emits the ultrasonic frequenciesf₁ and f₂ as a new wave form propagated at the face 78 of the speaker76. This new wave form interacts within the nonlinear medium of air togenerate the difference frequency 96, as a new sonic or subsonic wave.

The audio system 70 is able to function as described because thecompression waves corresponding to f₁ and f₂ interfere in air accordingto the principles of acoustical heterodyning. Acoustical heterodyning issomewhat of a mechanical counterpart to the electrical heterodyningeffect which takes place in a non-linear circuit. For example, amplitudemodulation in an electrical circuit is a heterodyning process. Theheterodyne process itself is simply the creation of two new waves. Thenew waves are the sum and the difference of two fundamental waves.

In acoustical heterodyning, the new waves equaling the sum anddifference of the fundamental waves are observed to occur when at leasttwo ultrasonic compression waves interact or interfere in air. Thepreferred transmission medium of the present invention is air because itis a highly compressible medium that responds non-linearly underdifferent conditions. This non-linearity of air is possibly what enablesthe heterodyning process to take place without using an electricalcircuit. However, it should be remembered that any compressible fluidcan function as the transmission medium if desired.

As related above, the acoustical heterodyning effect results in thecreation of new compression waves corresponding to the sum and thedifference of ultrasonic waves f₁ and f₂. The sum is an inaudibleultrasonic wave which is of little interest and is therefore not shown.The difference, however, can be sonic or subsonic, and is shown as acompression wave 96.

FIG. 5 also lists as a component of the system 76 a means for combiningsignals 92. This device performs the function of modifying theultrasonic wave trains being generated by the ultrasonic signal source90. This modification consists of the means 92 for combining signals bycombining a first ultrasonic signal 91 with an electrical signal 82,representing the new compression wave 96 to be generated.

The method of combining signals 91 and 82 in the present invention ispreferably accomplished through amplitude modulation. Therefore themeans for combining signals in the first embodiment is an amplitudemodulator 92. This yields a carrier wave modulated with upper and lowersidebands. In this invention, one or both sidebands may be used. One ofthe sideband may be canceled by use of filtering circuitry 97.

An important feature of the present invention is that the base frequencyand single sideband are propagated from the same transducer face 78.Therefore the component waves are perfectly collimated. Furthermore,phase alignment is at maximum, providing the highest level ofinterference possible between two different ultrasonic frequencies. Withmaximum interference insured between these waves, one achieves thegreatest energy transfer to the air molecules, which becomes the"speaker" radiating element in a parametric speaker.

While researching this novel form of sound generation, the presentinventors discovered an unexpected and somewhat surprising absence ofaudio feedback, although it is not yet precisely understood whyparametric processing of the original audio signal 82 eliminates audiofeedback. Experimental efforts have confirmed this result. Indeed, theaudio system 70 described in the preferred embodiment of the presentinvention does not suffer from the effects of audio feedback.

It is envisioned that there are other useful applications of theprinciple learned from the elimination of audio feedback using aparametric speaker system. One important principle is demonstrated in analternative embodiment of the present invention.

For example, FIG. 6 shows an alternative embodiment which includes anactive microphone 102, the processing circuitry 104 of the parametricspeaker system shown in FIG. 5, and a conventional speaker 106 insteadof the parametric speaker of the preferred embodiment. The conventionalspeaker 106 generates audible sound directly, whereas the parametricspeaker 76 does so indirectly. Accordingly the principles of operationof the preferred embodiment can also apply to an audio system which doesnot utilize a parametric speaker as the final output audio source.

It is important to note that without additional processing, theprocessing circuitry 74 (FIG. 5) used to prepare the original audiosignal 82 for emission from the parametric speaker 76 will not result inan audible signal from the conventional speaker 106. This is because theconventional speaker is not capable of generating ultrasonic frequencieslike the parametric speaker 76. Consequently, once the original audiblesignal 82 is amplitude modulated onto the ultrasonic base carrierfrequency, the audible signal 82 can be recovered by demodulation withinan acoustic heterodyning system.

It is to be understood that the above-described embodiments are onlyillustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the spiritand scope of the present invention. The appended claims are intended tocover such modifications and arrangements.

What is claimed is:
 1. A system for eliminating audio feedback,comprising:at least one speaker member; at least one microphone meansfor detecting at least one sonic frequency emitted from the speakermember and for generating an electrical signal representative of the atleast one sonic frequency; processing means coupled to the microphonemeans for receiving the electrical signal wherein the processing meanscomprises the following parametric speaker components in which allsignals received from the microphone are processed as a single wave formfor parametric output through said parametric speaker components; a)ultrasonic frequency/generating means for generating a first ultrasonicfrequency; b) modulating means coupled to the ultrasonic frequencygenerating means and the processing means for generating a new wave formelectronically which includes the at least one first and secondultrasonic frequency having a difference in value equal to the at leastone sonic frequency; and c) at least one ultrasonic frequency emittercoupled to the modulating means and the speaker member for concurrentlypropagating from a common emitter face (i) the first ultrasonicfrequency and (ii) the at least one second ultrasonic frequency whichinteracts with the first ultrasonic frequency within a compressibletransmission medium to thereby generate the at least one sonic frequencyfree of audio feedback.
 2. The system for eliminating audio feedback asdefined in claim 1 wherein the at least one microphone means fordetecting the at least one sonic frequency and generating an electricalsignal representative of the at least one sonic frequency is comprisedof an active microphone.
 3. The system for eliminating audio feedback asdefined in claim 1 wherein the modulating means further comprises anultrasonic frequency signal generator which generates and transmits thesecond ultrasonic frequency to the at least one ultrasonic frequencyemitter and to the modulating means, to thereby generate the at leastone sonic frequency.
 4. The system for eliminating audio feedback asdefined in claim 1 wherein the at least one sonic frequency is a livesound source.
 5. The system for eliminating audio feedback as defined inclaim 4 wherein the live sound source is selected from the group of livesound sources consisting of speech, singing, musical instruments andother sonic sound generating devices.
 6. The system for eliminatingaudio feedback as defined in claim 1 wherein the at least one ultrasonicfrequency emitter is comprised of ultrasonic acoustical transducers. 7.A system as defined in claim 1, wherein the modulating means furthercomprises mixing means for electronically combining the first and secondultrasonic frequencies to form a single new wave form representing thesum of the first and second ultrasonic frequencies, said systemincluding means for transmitting the single new wave form to the commonemitter face for propagation into air.
 8. A system as defined in claim1, wherein the modulating means comprises means for amplitude modulatingthe first ultrasonic frequency with the at least one sonic frequency togenerate the second ultrasonic frequency as at least one sideband to thefirst ultrasonic frequency.
 9. The system as defined in claim 8, furthercomprising filtering means coupled to the amplitude modulating means foreliminating one of the sidebands.
 10. The system as defined in claim 1wherein the ultrasonic frequency generating means (i) generates andtransmits the second ultrasonic frequency to the at least one ultrasonicfrequency emitter, and (ii) transmits the second ultrasonic frequency tothe modulating means and wherein the modulating means includes inputmeans for mixing audible sounds with the second ultrasonic frequency asupper and lower sidebands.
 11. A method for eliminating audio feedbackfrom an audio system generating at least one sonic frequency, saidmethod comprising the steps of:(1) receiving at a microphone the atleast one sonic frequency as a total audio signal to be processed at aninput of the audio system; (2) processing the total audio signal,including the at least one sonic frequency by modulating the total audiosignal with an ultrasonic carrier frequency to generate a new wave formelectronically to thereby prepare it for emission from a parametricspeaker; and (3) emitting the ultrasonic frequency carrier wave combinedwith the at least one sonic frequency from the parametric speakerwithout bypassing any portion of the signal to a nonparametric speakerto thereby generate the at least one sonic frequency as an audible soundfree of audio feedback.
 12. The method for eliminating audio feedback asdefined in claim 11 wherein the step of receiving the at least one sonicfrequency at an input of the audio system further comprises receivingthe at least one frequency at an active microphone.
 13. The method foreliminating audio feedback as defined in claim 11 wherein the step ofprocessing the at least one sonic frequency further comprises the stepof amplitude modulating the at least one sonic frequency onto theultrasonic frequency carrier wave.
 14. The method for eliminating audiofeedback as defined in claim 11 wherein the step of processing the atleast one sonic frequency further comprises the steps of concurrentlypropagating from a common emitter face of the parametric speaker (i) asecond ultrasonic wave train having a second ultrasonic frequency and(ii) a first ultrasonic wave train having the ultrasonic frequencycarrier wave which interacts with the second ultrasonic wave trainwithin a compressible transmission medium.
 15. The method foreliminating audio feedback as defined in claim 14 wherein the step ofprocessing the at least one sonic frequency further comprises the stepof electronically combining the first and second ultrasonic wave trainsto form a single new electronic wave form.